//获取键盘数字
function insert(num) {
  document.form.textview.value = document.form.textview.value + num;

}
//=
function equal() {
  var exp = document.form.textview.value
  if (exp) {
    // eval计算
    document.form.textview.value = eval(exp)
  }
}
//C
function clean() {
  document.form.textview.value = ""
}
// back
function back() {
  var exp = document.form.textview.value
  // substring 减去一位
  document.form.textview.value = exp.substring(0, exp.length - 1)

}